Conventionally, a shading film is formed on a discharge lamp, a tungsten halogen lamp or the like. An example of a discharge lamp will be described below. A discharge lamp having a wattage as low as a lamp power of 35 W is put to practical use. Since the discharge lamp is small and has a high efficiency, it is used for an automobile headlight, a light source for the back light of a liquid crystal projector, or the like.
When the discharge lamp is used for an automobile headlight or a light source for the back light of a liquid crystal projector, the discharge lamp is combined with a reflecting mirror. In recent years, a discharge lamp using quartz, which cuts off ultraviolet rays, for an outer tube to prevent the reflecting mirror from deteriorating due to ultraviolet rays emitted from the discharge lamp is known. In general, in order to achieve a proper light distribution by combining the reflecting mirror and the discharge lamp, the position of the light-emitting portion, that is, the arc, should be controlled with a very high precision with respect to the reflecting mirror. However, since the arc, which is the light-emitting portion of the discharge lamp, is affected by such factors as the shape of the arc tube, pressure, tube voltage, and tube current, it is difficult to control the position of the light-emitting portion mechanically in the same manner as the filament of a bulb or the like.
Accordingly, a method for obtaining a precise light distribution by forming a shading film on the outer tube and optically cutting a part of the arc whose position is difficult to control is proposed. With this method, the light distribution depends on the accuracy of the position of the shading film rather than the arc. Therefore, it is necessary to coat the outer tube with the shading film with a good positional accuracy.
Conventional lamps have an arc tube surrounded by an outer tube. An outer lead extends from each electrode to each contact of a base to which the neck-shaped portion of the arc tube is fixed. The power supply line of one outer lead extends along the outer surface of the outer tube. The outer tube is coated with a shading film near its neck-shaped portion and on the side distant from the power supply line by using a brush or an ink jet. Also, the outer tube is coated with a belt-shaped shading film at both ends of the discharge path between the electrodes and on the side facing the power supply line (Japanese Patent Application No. (Tokuhyo Hei) 9-500489).
However, with such a method for forming a shading film by using a brush or an ink jet, coating with a shading film must be performed by a machine. Therefore, the machine cost and the coating time are necessary. In addition, the control of a coating material for the shading film and the coating step are complicated.
Furthermore, with the above method, variations in the thickness of the shading film occur easily during coating. Also, since the coating material is a liquid, the thickness of the border portion of the formed shading film is smaller than that of the central portion of the shading film. Therefore, the shading property of the border portion of the shading film and its vicinity after firing deteriorates. That is, there is a problem that the edge of the border portion of the shading film has a gentle slope structure. In the lamp that controls the light distribution by the shading film, the positional accuracy and linearity of the border portion of the shading film coated on the outer tube affect the light distribution significantly. Therefore, it is necessary to control the shading film, especially the border portion of the shading film, with a good positional accuracy.